The present invention relates generally to synchronous transmission systems, and more specifically to the insertion of a virtual tributary path idle (VTPI) code into a multiplex signal.
According to CCITT Recommendation G709, it is specified that a virtual tributary path idle (VTPI) signal be inserted to a STS-1 (synchronous transport signal level-1) frame if at least one of the virtual tributaries of the frame carries no data signals. A conventional technique for the VTPI insertion involves the use of a set of a VTPI generator and a selector for each virtual tributary for generating a VTPI and combining it with data bits.
Since the current approach needs as many sets of VTPI generator and selector as there are virtual tributaries, it results in circuit complexity and an increase in terminal equipment size and cost.